Add a cautionary note on the default Seq/Traversable/Iterable types accepting the mutable version

> ...in your own methods and constructors, *receive the most generic collection type appropriate*. This typically boils down to one of the above: `Iterable`, `Seq`, `Set`, or `Map`.

It's worth mentioning that the default types in scope for `Seq`, `Traversable` and `Iterable` are the base versions defined in `scala.collection`, which accept the mutable versions as well. This is especially important in the context of the above quote – exposing them in method signatures.